1. Field of the Invention
The present invention relates to a ferroelectric liquid crystal device, for use in a liquid crystal display or a liquid crystal optical shutter. More particularly, the present invention is concerned with a liquid crystal device having improved display characteristics because of an improvement of the state of liquid crystal molecular alignment.
2. Related Background Art
Clark and Lagerwall have proposed a display device which uses the refractive index anisotropy of ferroelectric liquid crystal molecules and controls light ray transmission by combination with a polarizer (see Japanese Patent Laid-Open Application No. 56-107216 and U.S. Pat. No. 4,367,924). This ferroelectric liquid crystal commonly has a chiral smectic C phase (Sm*C) or H phase (Sm*H) of non-spiral structure in a specific temperature range and when under such state, assumes either a first or a second optically stable state in response to an applied electric field. The liquid crystal also maintains that state when no electric field is applied, in other words, the liquid crystal is bistable. This ferroelectric liquid crystal also has a quick response to changes in electric fields, and is expected to be utilized in a high-speed memory type display device. In particular, in view of its function, it is expected to be useful for a high resolution large screen display device.
In order for an optical modulation device applying such a bistable liquid crystal to exhibit desired drive characteristics, a liquid crystal disposed between a pair of parallel substrates must be in the state of molecular orientation such that conversion between the two stable states may effectively occur, independently of an applied state of electric field.
In the case of a liquid crystal device in which the double refraction of a liquid crystal is utilized, the transmittance under crossed nicols is expressed by: ##EQU1## wherein I.sub.O : incident light intensity,
I: transmitted light intensity, PA1 .theta.: tilt angle PA1 .DELTA.n: refractive index anisotropy PA1 d: film thickness of a liquid crystal layer, PA1 .lambda.: wavelength of incident light.
Thus, the tilt angle .theta. in the above non-spiral structure is present as an angle in the direction of an average molecular axis of a liquid crystal molecule twistingly oriented in the first and second states of alignment. According to the above expression, the transmittance reaches a maximum when the tilt angle .theta. is an angle of 22.5.degree., and hence a tilt angle .theta. in the non-spiral structure realizes the bistable state is required to be as near as possible to 22.degree..
It is desirable, for instance, to use a method by which a molecular layer formed of a plurality of molecules that form a smectic liquid crystal can be monoaxially aligned along its normal using a manufacturing process which can be carried out simply. The method as disclosed, for example, in U.S. Pat. No. 4,561,726 is known as a method of aligning a ferroelectric liquid crystal, in particular, a chiral smectic liquid crystal of non-spiral structure.
However, in instances in which alignment methods hitherto used, in particular, those methods using a polyimide film subjected to rubbing, the following problems occur.
Experiments made by the present inventors revealed that a tilt angle .theta. (the angle shown in FIG. 3 described later) in a ferroelectric liquid crystal of non-spiral structure obtained using a conventional rubbing polyimide alignment film becomes smaller than a tilt angle (the angle .theta. of 1/2 of a vertical angle of a trigonal pyramid shown in FIG. 2 described later) in a ferroelectric liquid crystal of spiral structure. In particular, the tilt angle .theta. was approximately 3.degree. to 8.degree. in general, and the transmittance at that time was approximately 3 to 5% at best.
Thus, according to Clark and Lagerwall, the tilt angle in a bistable ferroelectric liquid crystal should be the same angle as the tilt angle in a spiral ferroelectric liquid crystal. In practice, however, the tilt angle .theta. is smaller than the tilt angle .theta. due to the twisted orientation of liquid crystal molecules in the non-spiral structure. More specifically, in the non-spiral ferroelectric liquid crystal, liquid crystal molecules are twistingly oriented in a continuous form at a twist angle .delta., which is twisted from an axis of a liquid crystal molecule adjacent to an upper substrate toward en axis of a liquid crystal molecule adjacent to a lower substrate (i.e., in the direction of twisted orientation), with respect to the normal of each substrate. This is the reason why the tilt angle .theta. in the non-spiral structure becomes smaller than the tilt angle .theta. in the spiral structure.
In the instance where a chiral smectic liquid crystal is aligned using a conventional rubbed polyimide film, the polyimide alignment film is an insulating layer between an electrode and a liquid crystal layer. Hence, when a voltage with one polarity is applied so that the first optical stable state (for example, a white display) is switched to the second optical stable state (for example, a black display), a reverse electric field V.sub.rev with the other polarity is produced in the ferroelectric liquid crystal layer after removal of the application of this voltage with one polarity, often causing an after-image at the time of display. The above phenomenon of producing a reverse electric field is discussed in Yoshida Akio, "Switching Characteristics of SSFLC", A Collection of Drafts for Liquid Crystal Forum, p. 142-143, October, 1987.